Dry Low NOx (DLN) gas turbine engines include a can annular combustion arrangement where each can combustor includes a pilot burner and several main premix burners disposed circumferentially about the pilot burner. For each can combustor there is a main fuel nozzle that supplies one or more fuels to the main premix burners, and a pilot nozzle that supplies one or more fuels to the pilot burner. DLN engines produce 25 parts per million (PPM) NOx, or less. Ultra Low NOx (ULN) engines are an emerging class of engines that produce even lower levels of NOx than DLN engines.
DLN gas turbine engines are a result of an evolution of gas turbine engines where unwanted emissions have been reduced and efficiency increased by engine designs where the firing temperatures and operating pressures are ever increasing. The main burner fuel nozzle (a.k.a. support housing) is disposed in the compressed air manifold at an inlet end of the combustor where compressed air is at its greatest pressure, greatest temperature, and where the compressed air is undergoing a reversal of flow direction at the inlet end of the combustor. The high temperature and high pressure of the operating environment, as well as corrosive fuels, are known to cause stress corrosion cracking in the main fuel nozzle, which leads to limited life for the fuel manifold.
Concurrent with the need to survive in the relatively harsh DLN (as well as ultra low NOx (ULN) operating environment is a requirement that a fuel manifold of the main burner fuel nozzle be able to receive one or more fuel supplies and distribute them to several different fuel rockets, where there is one rocket for each premix main burner. The fuel rockets may further be divided into more than one stage. Further complicating the fuel manifold's design, in some embodiments the fuel manifold must be able to receive a second, different fuel and also distribute the second fuel to each rocket, also perhaps in more than one stage.
Conventionally, due to the complication of the fuel manifold, the required passages were machined into the fuel manifold. Milling, drilling, and welding-together the fuel manifold parts in order to create the complex channels resulted in stress risers where sharp corners were created, or where welds were located in regions of relatively high stress within the finished fuel manifold etc. In order to provide a fuel manifold that was strong enough to resist stress corrosion cracking long enough to provide a support housing with a viable lifespan, designers have used forged sub components and joined them together to form the fuel manifold. The fuel rockets were then welded to the forged fuel manifold. This technique has provided great flexibility in design, but it has a cost because the forged parts are more expensive, and machining it likewise expensive.
Complicating the matter still further is a need to provide for an expansion element on the main burner fuel nozzle to accommodate the relative thermal expansion of the internal fuel circuits. For example, in a dual fuel main burner nozzle, a fuel gas may be directed to an interior of the fuel rocket via one or more stages of fuel gas circuits. A fuel oil may also be directed through the fuel rocket and be ejected from the fuel rocket at a location proximate where the fuel gas is ejected. The fuel oil tube may be secured to the main burner nozzle and the fuel rocket ejection location, but the fuel rocket and the fuel oil tube often experience differential thermal expansion. Previously, this has been accommodated using a bellows type compensator built into the base of the fuel rocket. However, the thin plies of the bellows are highly susceptible to a number of failure modes, including stress corrosion cracking, cyclic failure, and rupture.
To overcome the foregoing problems and yet provide a main fuel nozzle having a reasonable service life designers have continued to seek stronger and stronger materials for the fuel manifold, and with this comes the attendant higher cost. Consequently, there remains room for improvement in the art.